feat: add citation functionality to LLM node context handling

[liugddx]

Close #27417

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Summary

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Checklist

• [ ] This change requires a documentation update, included: Dify Document
• [x] I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• [x] I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• [x] I've updated the documentation accordingly.
• [x] I ran dev/reformat(backend) and cd web && npx lint-staged(frontend) to appease the lint gods

[gemini-code-assist[bot]]

Summary of Changes

Hello @liugddx, I'm Gemini Code Assist[^1]! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request enhances the LLM node's context handling by introducing a citation mechanism. It allows the system to automatically embed source markers into the context provided to the LLM and instructs the LLM to utilize these markers in its responses. This feature aims to improve the transparency and verifiability of LLM outputs by directly linking generated content to its original sources.

Highlights

• New Citation Configuration: Introduced a new citation_enabled boolean field within the ContextConfig for LLM nodes, allowing users to explicitly enable or disable citation functionality.
• Contextual Citation Markers: Implemented logic to automatically append [position] citation markers to content retrieved from context when citation_enabled is active and position metadata is available. This ensures that source information is directly integrated into the context provided to the LLM.
• LLM Citation Instruction: Added a mechanism to inject a specific instruction into the LLM's prompt. This instruction guides the LLM to use the provided [position] markers when generating responses, promoting accurate source attribution.
• Comprehensive Unit Tests: Included a new test file with extensive unit tests covering various scenarios for the citation functionality, including cases with and without citations, missing position metadata, and string-based contexts, ensuring robustness and correctness.

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[liugddx]

pin~

[laipz8200]

I suggest that we go back to the original issue to discuss the implementation of this feature, rather than directly submitting a PR, as this would make code review difficult.